A lens is often used for refractive correction during a course of perimetry or other types of visual field testing of a patient. One example of refractive correction is a liquid trial lens (see for example U.S. Pat. No. 8,668,338, the contents of which are hereby incorporated by reference), which may be more sensitive to patient eye alignment than other trial lenses. Prior to beginning a perimetry test, a technician operating the perimeter typically tries to center a patient's eye at or near the z-axis (the axis of symmetry) of the perimeter's bowl. Live image(s) or video of the eye, captured using the instrument's camera, are displayed to the technician for patient alignment or test verification purposes. The technician sets an initial patient pupil position by adjusting the chin rest until the eye is determined to be centered in the displayed eye video. This initial position may or may not be centered on the refractive correction lens in use. Further, patients move around during the course of the testing. Due to these factors, the patient's eye is often misaligned with the correction lens, or becomes misaligned during the course of testing. This misalignment can result in 1) poor gaze monitor performance, 2) trial lens rim artifacts in the visual field report, and 3) degraded retinal sensitivity measurements due to stimulus blur induced by such decentration of the eye relative to the lens center.
Some perimeters use a set of fixed focus trial lenses for refraction correction. These lenses typically may not be as sensitive to decentration as is a liquid trial lens with regard to successful gaze monitor performance, but still can generate trial lens artifacts if the eye is not accurately aligned to the center of the lens. Thus, there is a need for an approach that can correctly position or align a patient's eye to the center of the refractive correction lens.